1. Field of the Invention
The present invention relates generally to antennas and more particularly to scanned antennas.
2. Description of the Related Art
Investigators around the world are working on the development of automotive radar systems for a variety of uses (e.g., adaptive cruise control and collision-avoidance). In addition to signal generating, receiving and processing circuits, a key element of these radar systems is a compact antenna that can cooperate with the generating circuits and radiate a scanned radar beam. Obstacles that are interrogated by the scanned beam generate an echo which is received by the antenna and coupled to the receiving and processing circuits.
For an automotive radar to be commercially viable, it must operate with low maintenance costs over a long lifetime and its elements are preferably based on well-developed technologies so as to reduce technical and schedule risks. Additionally, the radar must be light weight, low cost, and spatially compact while delivering good performance (e.g., high gain and low sidelobes). Space for an automotive radar is limited and, in particular, vertical space is particularly scarce. Accordingly, signal access to the antenna is preferably on its rear face to reduce the radar's vertical height.
One way of enhancing an antenna's side lobe performance is to increase its focal length but this process tends to increase the antenna's size. Antenna focal paths are, therefore, often spatially "folded" to reduce antenna volume. One conventional folding technique produces a Cassegrain antenna which illuminates a subreflector with a small radiator (e.g., a feed horn). Energy reflected from the subreflector then illuminates the antenna's primary reflector (the signal may also be collimated by passage through a lens). Antenna size is reduced because the focal path is folded twice. Although the feed antenna and the subreflector facilitate folding of the focal path, they block a portion of the path and degrade the antenna's gain and side lobe performance.
Various antenna structures have been developed to retain the benefits of focal path folding while reducing the degrading effects of a Cassegrain antenna.
In an exemplary antenna structure (e.g., see U.S. Pat. No. 4,220,957 issued Sep. 2, 1980 to Britt), a feedhorn protrudes through an aperture in a main reflector which includes a first grid of wire conductors. Positioned in front of the main reflector is a subreflector formed from a second grid of wire conductors that are oriented at 45.degree. to the conductors of the first grid. Radiation from the feed horn is initially reflected from the subreflector to the main reflector. Upon reflection from the main reflector, the radiation is collimated and its polarization rotated so that it now passes through the subreflector.
In another exemplary antenna structure (e.g., see U.S. Pat. No. 5,455,589 issued Oct. 3, 1995 to Huguenin, et al.), radiation passes through an aperture in a twistreflector and is reflected from a transreflector back to the twistreflector. The radiation's polarization is rotated as it is reflected from the transreflector. Accordingly, the radiation now passes through the transreflector and is collimated by a lens.
Although these exemplary antennas eliminate the subreflector path blockage of a Cassegrain antenna, they respectively require a horn aperture in a main reflector and a twistreflector so that gain and side lobe performance is still degraded.
In another exemplary antenna structure (e.g., see U.S. Pat. No. 3,797,020 issued Mar. 12, 1974 to Roger, et al.), a polarization discriminator is positioned between a radiator and a subreflector. A main reflector is laterally spaced from the polarization discriminator. The subreflector and the main reflector include twist reflector structures. Accordingly, radiation from the radiator initially passes through the polarization discriminator but is reflected from it to the main reflector after reflection and polarization rotation by the subreflector. The main reflector reflects the radiation and again rotates the polarization so that the radiation now passes through the polarization discriminator.
Although this arrangement removes the subreflector path blockage of a Cassegrain antenna and the aperture requirement of Britt and Huguenin, et al., it precludes signal access at an antenna rear face which inhibits its integration into an automotive radar system.